The invention relates to wear-resistant hardfacings for movable parts and especially to hardfacings for rotors of progressing cavity pumps.
Progressing cavity pumps have been used in water wells for many years. More recently, such pumps have been found well suited for the pumping of viscous or thick fluids such as crude oil laden with sand. Progressing cavity pumps include a stator which is attached to a production tubing at the bottom of a well and a rotor which is attached to the bottom end of a pump drive string and is made of metallic material, usually high strength steel. The rotor is usually electro-plated with chrome to resist abrasion, but the corrosive and abrasive properties of the fluids produced in oil wells frequently cause increased wear and premature failure of the pump rotor. Since it is important for efficient operation of the pump that a high pressure differential be maintained across the pump, only small variations in the rotor""s dimensions are tolerable. This means that excessively worn rotors must be replaced immediately. However, replacement of the rotor requires pulling a whole pump drive string from the well which is costly, especially in the deep oil well applications which are common for progressing cavity pumps. Consequently, pump rotors with increased wear resistance and, thus, a longer service life are desired to decrease well operating cost.
Various hardfacing methods have been used in the past to increase the wear resistance of metal surfaces. Hardfacings consisting of a thin layer of metal carbide applied by conventional thermal spraying techniques are the most commonly used due to the extreme hardness of the coating achieved. However, although this type of hardfacing works well when in friction contact with a metal surface, surfaces so coated have a roughness which makes them unacceptable for use in progressing cavity pump applications. The surface roughness of the metal carbide hardfacing is due to the grainy structure of the hardfacing structure which is caused by the individual sprayed-on metal carbide particles. This roughness results in excessive wear of the progressing cavity pump stator which is made of an elastomeric material, most often rubber. Polishing of the metal carbide hardfacing to overcome this problem is theoretically possible, but cannot be done economically due to the extreme hardness of the material. Thus, an economical hardfacing for progressing cavity pump rotors is desired which increases the surface life of the rotor without increasing stator wear. In particular, a hardfacing is desired which provides the surface hardness and wear characteristics of a metal carbide that is substantially insoluble in corrosive solutions found in wells.
Coating a metal component with a thin layer of a ceramic material or another metal is known. One primary purpose of a coating process is to protect the surface of a fragile metal product or substrate from abrasion or thermal degradation (i.e., melting) or oxidation by coating it with a more abrasion resistant and thermal degradation resistant material. Recently, various ceramics having high abrasion resistance or high oxidation resistance characteristics have been used to coat metal substrates. One method for applying a ceramic coating to the substrate is by spraying the ceramic coating onto the substrate.
Early equipment used for the spray-coating process, which typically is called flame spraying, included a wire-type flame sprayer. Flame spraying involves heating a heat fusible material, such as metal, to the point where it can be atomized and propelled through the gun onto the surface to be coated. The heated particles strike the surface and bond to it. In the typical flame spray gun, the acetylene and oxygen act as the fuel and combustion gas, respectively, creating the flame. Flame spraying includes oxyacetylene torch spraying. Examples of coatings produced by the flame spraying gun process are found in Ingham, H. S. and A. P. Shepard, Flame Spray Handbook, Vol. II (Metco Inc.)(2d ed 1964). The protective coatings that can be applied this way are limited to those materials that can be formed into a wire or rod. Commercially available flame spray guns also permit the use of a wide variety of metals, alloys, ceramics and cements which can be ground into a relatively fine powder to coat the object. However, high melting point materials are merely cemented by a matrix of material which can be melted in the flame plume. The typical flame spray gun is designed to apply self-fluxing alloys, self-bonding alloys, as well as oxidation-resistant alloys. The flame spray gun utilizes combustion to produce the necessary heat to melt the coating material. Other heating means such as electric arcs and resistance heaters may also be used in a flame spray gun.
In a plasma spray gun, the primary plasma gas is generally an inert gas such as nitrogen or argon. The gas mixture is heated by passing between electrodes with a high voltage discharge. A powder reservoir is attached to the gun and an a water cooler may be attached to the gun to prevent over-heating. Some metal powders require a triggered vibrator to maintain powder movement from the powder reservoir to the gun. The gun can either be xe2x80x9chand-heldxe2x80x9d or attached to a lathe for larger work, which rotates the metal component to be coated. Typically, the gun is perpendicular to the surface of the rotating object to be sprayed.
The plasma spray gun is the most versatile thermal spraying technique and produces enough heat to plasticize ceramic powder particles. The high thermal efficiency of the plasma spraying gun makes it possible to spray refractory materials at rates and deposit efficiencies which make the coatings economically feasible. The plasma spray technique can produce plume temperatures of 20,000 to 30,000 degrees and velocities of up to Mach 2.
However, ceramic coatings may be porous and may not afford much oxidation or corrosion protection to the base material. Therefore an undercoat of an oxidation-resistant or corrosion-resistant metal or alloy may be used between the base material and the ceramic coating.
Typically, ceramic coatings having high thermal resistance, have a lower wear resistance, while ceramic coatings having a high wear resistance have a low thermal resistance. The general reason for this relationship is that ceramic coatings having a high thermal resistance typically are more sponge-like and have a higher void content allowing thermal dissipation yet allowing easier abrasion, while ceramic coatings having a high abrasion resistance have a lower void content, thus reducing abrasion while at the same time lowering the heat dissipation properties.
Other melting spraying techniques are High Velocity Oxygen Fuel (HVOF) and Detonation Gun (D-gun).
In the HVOF technique, oxygen and a combustible fuel, either a gas or a liquid, are continuously injected into a combustion chamber and continuously ignited. The combustion gases are directed down a barrel and form a plume at the exit. The metal powder is injected into the plume axially in the barrel. This technique permits more efficient mixing of the metal powder in the plume, and may achieve plume velocities of up to Mach 3. The high velocity results in a coating having low porosity and permeability, and a compression coating is achieved which is more resistant to cracking if the part flexes. However, the lower temperature of the plume limits the use with ceramics.
In the D-gun, oxygen and a combustible gas are injected into a combustion chamber in an explosive mixture with a metal powder. The mixture is detonated and the combustion gases and metal are accelerated down a long barrel. This technique produces a high velocity plume compound to other metal spraying techniques, and a lower temperature than plasma and HVOF spraying.
The present invention is intended to included the use of any suitable thermal spraying technique, but plasma spraying is preferred.
U.S. Pat. No. 4,671,740 issued Jun. 9, 1987 to Ormiston et al. states that flame spraying a powder to produce a ceramic coating material on a metal substrate member results in porous coatings of low density that are not sufficiently abrasion resistant. U.S. Pat. No. 4,671,740 teaches the application of many small ceramic tiles that are bonded with an organic plastic adhesive to internal pump surfaces which is a complex, and time consuming process.
Therefore, one skilled in the art will appreciate that there is a need for more durable pump rotors for progressive cavity pumps and for a method for the ceramic coating of metal substrates which results in improved thermal resistance and improved wear resistance. The present invention now provides a hardfacing for a progressing cavity pump rotor which reduces problems of stator wear and corrosion experienced in progressing cavity pumps having rotors with metal carbide hardfacings.
U.S. Pat. Nos. 5,645,896 and 5,498,142 issued on Jul. 8, 1997 and Mar. 12, 1996 respectively to Robert A. R. Mills and commonly assigned to the present applicant disclose a hardfacing for downhole progressing cavity pumps and a method for producing the same. The hardfacing consists of a metal carbide layer applied to the ferrous pump rotor body by way of plasma spraying and a top layer of metallic material having a lower hardness than the metal carbide. The metal carbide layer has a grainy surface with a plurality of peaks and intermediate depressions, the peaks being formed by metal carbide grains at the surface of the metal carbide layer. The thickness of the top layer is adjusted such that the depressions between the peaks of the metal carbide layer are completely filled thereby providing the rotor with a metal carbide hardfacing of significantly reduced surface roughness. In the process of the invention, the pump rotor, which may be provided with a molybdenum bonding layer, is plasma coated with the metal carbide and the resulting metal carbide layer is covered with the metallic material top layer. The top layer is polished either until the dimensions thereof are within the tolerances acceptable for the finished rotor or until a majority of the peaks of the metal carbide layer are exposed.
Previously, it was believed that a ceramic would not function well sandwiched between two metal layers. It has now been found that a ceramic sandwiched between metals provides good resistance to abrasion and acts as a barrier to corrosion.
It would be advantageous to provide a hardfacing that is an improvement over metal carbides, in particular tungsten carbide, In addition, it would be advantageous to provide a hardfacing that has improved corrosion resistant/chemical resistant properties than metal carbides.
It has now been found that by hardfacing a downhole progressing cavity pump with a ceramic provides a rotor surface of greater durability to wear and corrosion.
It is an object of the invention to provide a progressing cavity pump of increased service life having a ceramic coating.
It is yet another object of the invention to provide an economical ceramic hardfacing for a progressing cavity pump rotor which has a low surface roughness.
These and other objects which will become apparent from the following are achieved with a hardfacing for a progressing cavity pump rotor in accordance with the invention. The hardfacing includes a layer of hard wearing ceramic bonded to the metal body of the rotor and may be overlaid by a top layer of a softer metallic material, either a pure metal or a metal alloy, which is polished more readily than the ceramic coating. Such a top layer may be applied at sufficient thickness to fill in the roughness of the ceramic layer or completely cover the first layer, and may be subsequently polished to a smooth finish having dimensions within desired tolerances. Preferably, the top layer is polished until a majority of the peaks of the grainy ceramic layer are exposed. This provides the rotor with a running surface which has the hard wearing characteristics but not the surface roughness of a pure ceramic coating, since the grainy surface structure of the ceramic layer is filled in by the metallic material of the second layer.
In one embodiment of the ceramic is material is a metal oxide, preferably alumina. In another embodiment, the ceramic material is applied by plasma spraying.
In one aspect the invention provides a pump rotor for a progressing cavity pump comprising a rotor body made of a ferrous metal and a coating on the ferrous metal comprising a ceramic metal oxide layer.
In another aspect, the invention provides a pump rotor for a progressing cavity pump comprising: (i) a rotor body made of a ferrous metal; (ii) a layer of a ceramic material plasma sprayed onto the body to form a ceramic layer, the ceramic layer having a grainy surface with a plurality of peaks and intermediate depressions, the peaks being formed by ceramic grains at the surface of the ceramic layer; and optionally, (iii) a top layer of metallic material bonded to the ceramic layer, the thickness of the top layer adjusted such that the depressions between the peaks of the ceramic layer are filled while a majority of the peaks are exposed at the surface of the rotor, thereby providing the rotor with a ceramic hardfacing.
The invention further provides a ceramic hardened metal surface comprising: (i) a ferrous metal body; (ii) a layer of a ceramic material plasma sprayed onto the ferrous metal body to form a ceramic layer, the ceramic layer having a grainy surface with a plurality of peaks and intermediate depressions, the peaks being formed by ceramic grains at the surface of the ceramic layer; and (iii) a top layer of metallic material bonded to the ceramic layer, the thickness of the top layer adjusted such that the depressions between the peaks of the ceramic layer are filled while a majority of the peaks are exposed at the surface of the rotor, thereby providing the metal body with a ceramic hardfacing.
The present invention also extends to a method of hardfacing a rotor for a progressing cavity pump having a ferrous metal rotor body comprising the step of: (i) plasma spraying a ceramic material onto the rotor body to form a ceramic layer on the rotor body having a grainy surface with a multiplicity of peaks and intermediate depressions, the peaks being formed by ceramic grains at the surface of the ceramic layer. Preferably the method further comprises the step of (ii) applying a metallic material top layer onto the ceramic layer at such a thickness that it substantially covers the ceramic layer; and (iii) polishing the top layer until a majority of the peaks of the ceramic layer are exposed.
In one embodiment, the top layer is of sufficient thickness to completely cover the ceramic layer and is made of a pure metal or a metal alloy. In addition, a molybdenum layer is applied directly onto the rotor body and prior to application of the ceramic layer to increase the bonding of the latter to the rotor body. The ceramic layer is preferably applied at such a thickness that the dimensions of the ceramic layer are within the tolerances selected for the finished rotor.
In a preferred economical embodiment, the top layer is not polished until the majority of peaks of the ceramic layer are exposed. The ceramic layer is applied so that its dimensions are within the selected tolerances for the finished rotor. The top layer is polished to achieve a smooth surface and only until the interference between the finished rotor and the stator is within acceptable limits. The rotor is put into service whereby the top layer is subjected to the usual wear experienced with conventional rotors. Then once the top layer is worn to the point where a majority of the peaks of the ceramic layer are exposed, the interference fit between the rotor and the stator is still satisfactory since the dimensions of the ceramic layer are within the selected tolerances for the finished rotor.
The ceramic material is preferably selected from among the ceramics formed from aluminium, boron, silicon, or titanium and the metallic material of the top layer is preferably selected from among chromium, molybdenum and nickel and alloys thereof. In the preferred embodiment, the ceramic layer is made of aluminium oxide or alumina and the second layer is made of chromium/molybdenum alloy or nickel/chromium alloy. Most preferably, the ceramic layer is presently achieved with an aluminium oxide layer having a thickness of 50 to 125 xcexcm (microns/micrometers) and overlayed with a nickel/chromium layer of 75 to 150 xcexcm.